P-Grid

1 Self-organizing Structured P2P Systems In the P2P community a fundamental distinction is made among unstructured and structured P2P systems for resource location. In unstructured P2P systems in principle peers are unaware of the resources that neighboring peers in the overlay networks maintain. Typically they resolve search requests by flooding techniques. Gnutella [9] is the most prominent example of this class. In contrast, in structured P2P systems peers maintain information about what resources neighboring peers offer. Thus queries can be directed and in consequence substantially fewer messages are needed. This comes at the cost of increased maintenance efforts during changes in the overlay network as a result of peers joining or leaving. The most prominent class of approaches to structured P2P systems are distributed hash tables (DHT), for example Chord [17]. Unstructured P2P systems have generated substantial interest because of emergent globalscale phenomena. For example, the Gnutella overlay network exhibits the following characteristics [15]: 1. The network has a small diameter, which ensures that a message flooding approach for search works with a relatively low timeto-life (approximately 7). 2. The node degrees of the overlay network follow a power-law distribution. Thus few peers have a large number of incoming links whereas most peers have a very low number of such links. These properties result from the way Gnutella performs network maintenance: each peer maintains a fixed number of active links. Using the network maintenance protocol a peer discovers new peers in the network by flooding discovery